Please help I have to draw the cross and dot diagrams for (a) H2O, (b) H3O+, (c) H4O2+ and H2O2, I also have to name their shapes ans bond angles and explain any differences between bond angles on a,b and c, I am totally stumped

Well, to know the shapes of the compounds, you first have to draw the dot and cross diagrams and then figure out how many bond pairs and lone pairs there are around the central atom. For instance, in H2O, there are two bond pairs and two lone pairs around the Oxygen atom. So, the shape would be Bent(angular) with bond angles 104.5 degrees. The reason is because the electrons repel each other as far apart as possible(minimum repulsion).
Try out drawing the dot and cross for H3O+ and other compounds.